1. Field of the Invention
The present invention relates to the field of inkjet printing and, in particular, discloses an improved micro-electromechanical inkjet actuator.
2. Description of Related Art
Thermoelastic actuator inkjet nozzle arrangements are described in US Patent Applications Nos. U.S. Ser. No. 09/798,757, now U.S. Pat. No. 6,460,971, filed on Mar. 2, 2001 and U.S. Ser. No. 09/425,195, now U.S. Pat. No. 6,592,207, filed on Oct. 19, 1999 which are both co-owned by the present applicant and herein incorporated by cross reference in their entireties.
A first nozzle according to an embodiment of the invention described in that document is depicted in FIG. 1. FIG. 1 illustrates a side perspective view of the nozzle arrangement and FIG. 2 is an exploded perspective view of the nozzle arrangement of FIG. 1. The single nozzle arrangement 1 includes two arms 4, 5 which operate in air and are constructed from a thin 0.3 micrometer layer of titanium diboride 6 on top of a much thicker 5.8 micron layer of glass 7. The two arms 4, 5 are joined together and pivot around a point 9 which is a thin membrane forming an enclosure which in turn forms part of the nozzle chamber 10. The arms 4 and 5 are affixed by posts 11, 12 to lower aluminium conductive layers 14, 15 which can form part of the CMOS layer 3. The outer surfaces of the nozzle chamber 18 can be formed from glass or nitride and provide an enclosure to be filled with ink. The outer chamber 18 includes a number of etchant holes e.g. 19 which are provided for the rapid sacrificial etchant of internal cavities during construction by MEM processing techniques.
The paddle surface 24 is bent downwards as a result of the release of the structure during fabrication. A current is passed through the titanium diboride layer 6 to cause heating of this layer along arms 4 and 5. The heating generally expands the T1B2 layer of arms 4 and 5 which have a high Young's modulus.
This expansion acts to bend the arms generally downwards, which are in turn pivoted around the membrane 9. The pivoting results in a rapid upward movement of the paddle surface 24. The upward movement of the paddle surface 24 causes the ejection of ink from the nozzle chamber hole 21. The increase in pressure is insufficient to overcome the surface tension characteristics of the smaller etchant holes 19 with the result being that ink is ejected from the nozzle chamber hole 21.
As noted previously the thin titanium diboride layer 6 has a sufficiently high young's modulus so as to cause the glass layer 7 to be bent upon heating of the titanium diboride layer 6. Hence, the operation of the inkjet device is as illustrated in FIGS. 3–5. In its quiescent state, the inkjet nozzle is as illustrated in FIG. 3, generally in the bent down position with the ink meniscus 30 forming a slight bulge and the paddle being pivoted around the membrane 9. The heating of the titanium diboride layer 6 causes it to expand. Subsequently, it is bent by the glass layer 7 as to cause the pivoting of the paddle 24 around the membrane 9 as indicated in FIG. 4. This causes the rapid expansion of the meniscus 30 resulting in a positive pressure pulse and the general ejection of ink from the nozzle chamber 10. Next the current to the titanium diboride is switched off and the paddle 24 returns to its quiescent state resulting in a negative pressure pulse causing a general sucking back of ink via the meniscus 30 which in turn results in the ejection of a drop 31 on demand from the nozzle chamber 10.
By shaping the electrical heating pulse the magnitude and time constants of the positive pressure pulse of the thermoelastic actuator may be controlled. However, the negative pressure pulse remains uncontrolled. The characteristics of the negative pressure pulse become more influential for fluids of high viscosity and high surface. Accordingly it would be desirable if thermoelastic inkjet nozzles with tailored negative pressure pulse characteristics were available.
A further difficulty with some types of thermoelastic actuators is that it is not unusual for very high temperature actuators to induce temperatures above the boiling point of any given liquid on the bottom surface of the non-conductive layer. It is an object of the present invention to provide a thermoelastic actuator with a tailored negative pressure pulse characteristic.